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H
2
SO
4
Introduction
Objectives
HCL
CH
3
COOH
HNO
3
References
Conclusions
H
2
CO
3

Study the effect of various acid attacks on concrete
Understand the mechanism of each attack and the response of concrete
Study few cases highlighting different types of acid attacks studied
Compare the effects and mechanisms of different acid attacks, where applicable.

Concretes made of Portland cement (OPC) are highly alkaline with pH values normally above 12.5 and are not easily attacked by acidic solutions.
As the pH of the solution decreases the equilibrium in the cement matrix is being disturbed, and the hydrated cement compounds are essentially altered by hydrolytic decomposition which leads to the severe degradation of the technical properties of the material.
At pH values lower than 12.5 portlandite is the first constituent starting dissolution.

If pH decreases to values lower than stability limits of cement hydrates, then the corresponding hydrate loses calcium and decomposes to amorphous hydrogel
The final reaction products of acid attack are the corresponding calcium salts of the acid as well as hydrogels of silicium, aluminum, and ferric oxides
The solubility of Al
2
O
3
⋅ aq, and Fe
2
O
3 depends on the pH value of the acting solution, while SiO
2 in acidic solutions except in HF is insoluble

ACID ATTACK
WEAK ACIDS
Acetic acid
Carbolic acid
Carbonic acid
Lactic acid
Phosphoric acid
Tannic acid
STRONG ACIDS
Hydrochloric acid
Sulphuric acid
Sulphurous acid
Nitric acid
Hydroflouric acid
Hydrobromic acid

Sulphuric acid attack causes extensive formation of gypsum in the regions close to the surfaces, and tends to cause disintegrating mechanical stresses which ultimately lead to spalling and exposure of the fresh surface.
Owing to the poor penetration of sulphuric acid, the chemical changes of the cement matrix are restricted to the regions close to the surfaces.

However, in some cases it is observed that deterioration process occurs accompanied by the scaling and softening of the matrix due to the early decomposition of calcium hydroxide and the subsequent formation of large amount of gypsum.
The chemical reactions involved in sulphuric acid attack on cement based materials can be given as follows:
Ca(OH)
2
+ H
2
SO
4
 CaSO
4
.2H
2
O
3CaO.2SiO
2
.3H
2
O + H
2
SO
4
 CaSO
4
.2H
2
O + Si(OH)
4

pH Range Effect
12.5 – 12 Calcium hydroxide and calcium aluminate hydrate dissolve and ettringite is formed
CSH phase is subjected to cycles of dissolution and re-precipitation
11.6 – 10.6
< 10.6
< 8.8
Gypsum is formed
Ettringite is no longer stable and decomposes into aluminum hydroxide and gypsum
CSH becomes unstable

Sulphur content versus distance from acid-exposed surface
(Reference 5)

ACI 210.1R-94
2

Source water has high sulphur content, both as sulphate or sulphide, and form hydrogen sulphide, H
2
S.
The hydrogen sulphide gas comes out of the solution and forms sulphuric acid in the air space.
Sulphuric acid is highly reactive and reacts with calcium compounds to form gypsum which causes the concrete to soften, ultimately leading to roof collapse.
Organic matter + SO
4
2 S 2+ H
2
O + CO
2
S 2+ 2H +  H
2
S
H
2
S + 2O
2
 H
2
SO
4

Nitric acid usually occurs in chemical plants producing explosives, artificial manure and similar products.
Nitric acid can be formed from the compounds and radicals of nitrates in the presence of water
3NO
2
+ H
2
O  2HNO
3
+ NO
Though HNO
3 is not as strong as H
2
SO
4
, its effect on concrete at brief exposure is more destructive since it transforms CH into highly soluble calcium nitrate salt and low soluble calcium nitro-aluminate hydrate

Nitric acid attack can be represented by the following equations;
2HNO
3
+ Ca(OH)
2
 Ca(NO
3
)
2
.2H
2
O
Ca(NO
3
)
2
.2H
2
O + 3CaO.Al
2
O
3
.8H
2
O 
3CaO.Al
2
O
3
. Ca(NO
3
)
2
.10H
2
O
Pavlik reported that the corroded layer developed by the action of nitric acid solution with concentrations ranging between 0.025 to 0.5 mol l -1 is soft, and porous with visible cracks.

Nitric acid attack is a typical acidic corrosion for shrinkage of the corroded layer due to leaching of highly soluble calcium nitrate.
Such volume contractions of the corroded layer, especially for the case of nitric acid, can result in the formation of visually observable cracks across the corroded layer.
In the presence of these cracks the transport rate of acid and corrosion products to and from the corrosion front increases and this accelerates the process of deterioration.

Variation of compressive strength with acid concentration
(mix ratio 1:1.5:3, W/C = 0.65)
(Reference 4)

Concrete in use in agricultural applications may be attacked by the silage effluents containing mainly acetic and lactic acid.
Acetic acid reacts with cement hydration products to form calcium acetate
2CH
3
COOH + Ca(OH)
2
 Ca(CH
3
COO)
2
+ 2H
2
O
2CH
3
COOH + C-S-H  SiO
2
+ Ca(CH
3
COO)
2
+ 2H
2
O

Attack by Acetic acid resembles the process of corrosion in nitric acid. However the growth of the corroded layer in solutions of acetic acid is relatively slower than that in the same concentrations of nitric acid solution.
The chemical composition of the corroded layer is different from that in nitric acid solution of the same concentration due to higher pH values of the acetic acid solution, and due to its buffering effect in corroded layer.
In lower concentrations of both acetic and nitric acid solutions, e.g. 0.025 mol l -1 , results in the formation of an additional zone, called as core-layer, which is relatively hard and located behind the corroded layer

Chemical compositions of the core layers in both acetic and nitric acid attacks are similar
Core-layer originates due to portlandite dissolution in unaffected part of the cement paste and diffusion of
Ca 2+ and OH ions towards corrosion zone where they meet the acid diffusing from the opposite direction
Formation of the core layer is noticeable only when the concentration of acid is low because in such a case the rate of diffusion of H + ions from the acidic solution is high enough to dissolve portlandite, but not sufficiently high to decalcify the other hydrates

The chemicals formed as the products of reaction between hydrochloric acid and hydrated cement phases are some soluble salts and some insoluble salts
Soluble salts, mostly with calcium, are subsequently leached out, whereas insoluble salts along with amorphous hydrogels, remain in the corroded layer.
Besides dissolution, the interaction between hydrogels may also result in the formation of some Fe-Si, Al-Si,
Ca-Al-Si complexes which appear to be stable in pH range above 3.5.

Ca(OH)
2
+ 2HCl  CaCl
2
+ 2H
2
O
The reaction essentially causes leaching of Ca(OH)
2 the set cement.
from
After leaching out of Ca(OH)
2
, C-S-H and ettringite start to decompose, with release of Ca 2+ to counteract the loss in
Ca(OH)
2 and the set cement starts to disintegrate accelerating the dissolution.
Ca
6
Al
2
(SO
4
)
3
(OH)
12
.26H
2
O  3Ca 2+ +2[Al(OH)
4
] +4OH +26H
2
O
3Ca 2+ +2[Al(OH)
4
] +4OH +12HCL  3CaCl2 + 2ALCL
3
+ 12H
2
O

There are few indications through experiments about the formation of Friedel’s salt, C
3
A.CaCl
2
.10H
2
O, by the action of CaCl
2
, formed due to reaction of HCL with CH and C
3
A
Hydrochloric acid attack is a typical acidic corrosion which can be characterized by the formation of layer structure.
Chandra divided the cross section of damaged prisms into three main zones; undamaged zone, hydroxide mixture zone or brown ring, and attacked zone.
By hydroxide mixture zone, he referred to a layer formed by undissolved salts seen as a dark brown ring.

Carbonic acid attack usually occurs in the case of buried concrete structures exposed to acidic ground water fro a long time
Atmospheric carbon dioxide absorbed by rain enters ground water as carbonic acid
Factors affecting the rate of carbonic acid attack are;
Quality of concrete
Concentration of aggressive carbon dioxide
External exposure conditions

When concrete is exposed to carbonic acid, a reaction producing carbonates take place which is accompanied by shrinkage
Limited carbonation of surface layer of concrete is known to seal the pores by forming calcium carbonate, which reduces the permeability and increases the strength of the carbonated layer.
However, continued carbonation may cause a reduction in alkalinity of the cement paste which can be a serious problem not only in de-passivation and corrosion of steel bars but also in dissolution of cement hydrates.

Grube and Rechenberg described that continued carbonation due to carbonic acid attack causes;
The transformation of calcium carbonate into soluble bicarbonate which is removed by leaching into the acidic solution and thus increasing the porosity.
H
2
CO
3
+ Ca(OH)
2
 CaCO
3
+ 2H
2
O
H
2
CO
3
+ CaCO
3
 Ca(HCO)
3
Decomposition of cement hydration products, leading to formation of gel-like layer consisting of hydrogels of silica, alumina and ferric oxide

ACI 210.1R-94

The Dworshak reservoir collects snow melt runoff and releases the pure water during the seasonal incubation and rearing phase of the hatchery production
There was high concentration of dissolved carbon dioxide in the collected water pH of the collected water was 6.5-7.4
Diagnosis of carbonic acid attack was accomplished by identifying the H
2
CO
3 to Ca(HCO
3
)
2 ratio contained in the water in contact with the cement.

In the case of sulphuric acid attack, although the formation of gypsum has been reported frequently, there is no agreement on its consequences
Attack by Acetic acid resembles the process of corrosion in nitric acid. However the growth of the corroded layer in solutions of acetic acid is relatively slower than that in the same concentrations of nitric acid solution
The chemical composition of the corroded layer is different from that in nitric acid solution of the same concentration due to higher pH values of the acetic acid solution

Though HNO
3 is not as strong as H
2
SO
4
, its effect on concrete at brief exposure is more destructive
Limited carbonation is found to be somewhat beneficial but continued carbonation reduces alkalinity of the cement paste which can be a serious problem not only in de-passivation and corrosion of steel bars but also in dissolution of cement hydrates

Mark G. Richardson, Fundamentals of Durable Reinforced
Concrete , 2002
Ali Allahverdi and Frantisek skvara, Acidic corrosion of hydrated cement based materials , 2000, Institute of chemical technology,
Department of glass and ceramics
Compendium of Case Histories on Repair of Erosion-Damaged
Concrete in Hydraulic Structures, ACI 210.1 R-94 (reapproved
1999)
Kolapo O. Olusola and Opeyemi Joshua , Effect of Nitric Acid
Concentration on the Compressive Strength of Laterized
Concrete, Vol. 2, No. 10, 2012
Emmanuel K. Attiogbe and Sami H. Rizkalla, Response of concrete to sulphuric acid attack , 1989, ACI Material journal,
Title no. 85-M46